Integrated Services Digital Network (ISDN) standards specify a subscriber "S" reference point (ISDN User-Network Interfaces-Layer 1 Recommendations contained in "CCITT I-Series Recommendations" which is incorporated herein by reference). Network Termination (NT) equipment can exist on either the subscriber or network side of the "S" interface and provides functions necessary for the operation of the access protocols by the network and essential functions for transmission. Terminal Equipment (TE) exists on the subscriber side of the "S" interface and provides functions necessary for the operation of the access protocols by the user. The CCITT reference points are conceptual showing the conjunction of two non-overlapping functional groups and may not correspond to a physical interface.
Depending on the placement of equipment with respect to the "S" interface, the equipment can be either a master or a slave in the sense that it is either a source of a clock synchronizing data movement over the interface or a receiver of a synchronizing clock, respectively.
In some instances, equipment resides between two clock sources, for example, it can receive a clock recovered from the network via the "S" interface and a clock recovered from a pulse code modulated (PCM) highway. Data may have to be moved between the "S" interface and the PCM highway, and the two data clocks will have the same or very nearly the same frequency. If they have different frequencies, data movement between the "S" interface and the PCM highway will result in occasional loss of data or doubling of data. When data is lost or doubled, because of different clock rates, data "slip" has occurred.
Typically, avoidance of data slip is performed by having a central circuit, separate from the equipment, which monitors the clocks from a number of equipment connected to the PCM highway and synchronizes the frequency of the highway. If the equipment is performing time-division multiplexing (TDM) of data from various sources and destinations, the synchronizing functions is separated from the multiplexing function, so that it is difficult to control data transfers through the multiplexer for anything but very small phase differences between the clocks. In addition, where the equipment is implemented by integrated circuitry, an extra pin is required to bring out the clock signal. Typically, pins are at a premium because of the complex functions performed by the integrated circuits, and their resultant need for numerous signals to be applied and conducted away from the circuit. Use of a pin for such an inessential signal is undesirable.